Various processes are known for producing alpha-, beta-unsaturated carboxylic acid esters. Some such processes incorporate oxidative steps, such as the vapor phase oxidation of propylene, isobutylene, tertiary butanol, methacrolein, acrolein, or isobutyraldehyde to afford acrylic or methacrylic acid followed by esterification to its respective (meth)acrylic ester. Others involve reaction of acetone cyanohydrin with sulphuric acid to form methacrylamide, which is then esterified with methanol to produce methyl methacrylate. Many, if not all, such processes are known to result in product mixtures which contain quantities of carbonyl-containing impurities such as aldehydes and ketones, for example, without limitation, benzaldehyde, biacetyl, furfural, protoanemonin, methacrolein, and acrolein. These impurities are undesirable because they may react with the alpha-, beta-unsaturated esters in subsequent reactions, they may interact with other reactants which are intended to react with the esters in subsequent reactions, they may react to form colored impurities, or they may directly inhibit subsequent reactions.
In particular, certain end users of methyl methacrylate (MMA) require biacetyl (2,3-butanedione) levels below 3.0 ppm by weight (wppm) to avoid yellowing in poly(methyl methacrylate) sheets. Furthermore, some uses of MMA require biacetyl levels to be below detectable limits. Thus, methods have been developed to reduce biacetyl levels during methyl methacrylate production. A preferred methodology for doing this utilizes ortho-phenylenediamine (oPD) as an additive for complexing with the biacetyl and forming a heavy component that boils much higher than MMA. Prior art (U.S. Pat. No. 4,668,818 and European Patent No. EP 0206230) describes the preferential addition of at least a 20-fold or greater molar excess of oPD relative to biacetyl. In the prior art, oPD is added to the MMA esterification reactor in the presence of a strong acid such as sulfuric acid. The use of amines for removal of carbonyl impurities such as aldehydes and ketones from alpha-, beta-unsaturated acids, such as (meth)acrylic acid, is known. Unfortunately, amines effective for reducing impurities from alpha-, beta-unsaturated acids are not necessarily effective for reducing or removing impurities from alpha-, beta-unsaturated esters. For example, aniline is highly effective in reducing carbonyl impurities from acrylic acid but has been found to be quite ineffective in reducing impurities in butyl acrylate. One reference, Japanese Kokai No. 52-23017 (“JK017”), discloses a process for purifying (meth)acrylic acids and esters by distilling in the presence of polyamines of type R1—NH—R—NH—R2. This method requires the use of neutral conditions and, when “R” is a phenylenic group, the method is effective only when the amino groups are immediately adjacent one to the other (i.e. in the 1, 2, or ortho position). Ortho adjacency is required in the JK017 disclosure because, as taught, the cyclic compound formed between the diamine and a carbonyl impurity cannot result from diaminophenylenes other than from those having the 1,2 relationship. Accordingly, meta- and para-phenylene diamines, for example, are specifically excluded, although it would be advantageous to use diamines such as these because of their low cost and availability.
U.S. Pat. No. 3,124,609 discloses the use of amines such as hydroxylamine and ortho-phenylenediamine for the removal of biacetyl from the MMA esterification reactor mix (i.e., the crude MMA product). The molar ratio of oPD:biacetyl used in the process described in U.S. Pat. No. 3,124,609 was 16:1, followed by a holding period of at least 30 minutes and neutralization of the acid.
U.S. Pat. No. 5,468,899 describes the use of amines including phenylenediamines, but not ortho-phenylenediamine, to remove carbonyl impurities from MMA. While biacetyl removal is not specifically addressed in U.S. Pat. No. 5,468,899, the process requires a neutralization step, and the process requires a holding step with a contact time of several hours prior to further distillation.
Another process is disclosed in U.S. Pat. No. 5,585,514 wherein non-aromatic diamines are added at the product column, during purification of the crude MMA, at a preferred diamine:biacetyl mole ratio of at least 10:1, in the presence of 1-2% methacrylic acid. This process requires a holding period of at least 10 minutes.
In a more recent publication in August 2009, a process has been disclosed wherein up to a 10-fold excess of ortho-phenylenediamine, based on biacetyl content, was added to crude methyl methacrylate prior to the first separations to purify the methyl methacrylate product. See Research Disclosure Database Number 544006, Research Disclosure Journal, August 2009.
The problem addressed by the present invention is the reduction of biacetyl from alpha-, beta-unsaturated esters, particularly (meth)acrylic esters, to provide high purity alpha-, beta-unsaturated esters in an efficient and low cost process.